4 ms·

If the Earth's magnetic field flips somewhat randomly every 1/2 million years, and there is randomness in that timing, how can it be a reliable measurement mech

If the Earth's magnetic field flips somewhat randomly every 1/2 million years, and there is randomness in that timing, how can it be a reliable measurement mechanism at 50 million years? 500 million years? 2 billion years?

njarboe 8y ago
From the sea-floor anomalies, we are pretty certain that the field is mostly a dipole (magnet like) during last 160 million years. The actual magnetic field direction at any one place at a specific time will vary from the ideal dipole field by up to around 30 degrees, but over thousands of years the average field direction is that of a dipole(mostly). So, to get a good idea of the true direction of the magnetic field at a given place on a continent in the past, one would want a decent number of measurements of different ages (but not enough time to pass so that tectonic motions are significant, say less than a few million years). The more measurements the more certain one is on the accuracy of the direction and a lot of Fischer statistics are used in this determination to understand how certain one knows the true direction. The further back we go the more unreliable and less certain we are that this is a reliable mechanism. The rocks get changed by chemical reactions or heat. The solid inner core grows, so the solid core size will change the dynamics of the field greatly. The whole mantle seems to have moved very quickly all at once durning some times, but so this was so long ago there are few rocks that have been studied that this strange possibility is still highly debated. If someone tells you about the magnetic field of the Earth at 2 billion years, they are just stating a speculative theory. The last 160 million years, we have a pretty good idea. If you want to learn in depth about this science, I recommend you check out a text book by Butler (1992) https://www.geo.arizona.edu/Paleomag/ https://www.geo.arizona.edu/Paleomag/ which is good intro textbook or Tauxe (2018) https://earthref.org/MagIC/books/Tauxe/Essentials/ https://earthref.org/MagIC/books/Tauxe/Essentials/ Which is more up-to-date and more in-depth. Or even chat online if you like. email is in my profile.

njarboe 8y ago

From the sea-floor anomalies, we are pretty certain that the field is mostly a dipole (magnet like) during last 160 million years. The actual magnetic field direction at any one place at a specific time will vary from the ideal dipole field by up to around 30 degrees, but over thousands of years the average field direction is that of a dipole(mostly). So, to get a good idea of the true direction of the magnetic field at a given place on a continent in the past, one would want a decent number of measurements of different ages (but not enough time to pass so that tectonic motions are significant, say less than a few million years). The more measurements the more certain one is on the accuracy of the direction and a lot of Fischer statistics are used in this determination to understand how certain one knows the true direction. The further back we go the more unreliable and less certain we are that this is a reliable mechanism. The rocks get changed by chemical reactions or heat. The solid inner core grows, so the solid core size will change the dynamics of the field greatly. The whole mantle seems to have moved very quickly all at once durning some times, but so this was so long ago there are few rocks that have been studied that this strange possibility is still highly debated. If someone tells you about the magnetic field of the Earth at 2 billion years, they are just stating a speculative theory. The last 160 million years, we have a pretty good idea. If you want to learn in depth about this science, I recommend you check out a text book by Butler (1992) https://www.geo.arizona.edu/Paleomag/ https://www.geo.arizona.edu/Paleomag/ which is good intro textbook or Tauxe (2018) https://earthref.org/MagIC/books/Tauxe/Essentials/ https://earthref.org/MagIC/books/Tauxe/Essentials/ Which is more up-to-date and more in-depth. Or even chat online if you like. email is in my profile.